1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a technique which can shorten a time until the normal display appears from the time of starting the liquid crystal display device in a low-temperature environment.
2. Description of the Related Art
A liquid crystal display device has been popularly used in various fields such as a computer terminal, a television receiver set, a mobile phone or the like. Although various kinds of displays are used in automobile, the application of the liquid crystal display device has been also spreading to these fields. The display used for the automobile is necessary to withstand a broad temperature environment. When the temperature of liquid crystal is lowered, a response speed corresponding to an electric field is lowered. Accordingly, at the time of starting the automobile during a winter season, the inside of the automobile is at a low temperature and hence, there exists a drawback that a considerable time is necessary until the liquid crystal display device becomes warm and the normal operation is acquired.
To cope with such a drawback, a technique which mounts a heater on a liquid crystal display device thus shortening time until the liquid crystal display device becomes warm is disclosed in JP-A-7-153559 (patent document 1) or JP-A-2006-201575 (patent document 2), for example.
On the other hand, the liquid crystal display device has a drawback that a viewing angle is not sufficient. Even when the liquid crystal display device is applied to an automobile, a display is not always positioned right in front of a person who watches the display and hence, it is preferable that the viewing angle is as wide as possible. Among various liquid crystal display devices, an IPS (In Plane Switching) liquid crystal display device which adopts a lateral electric field driving method exhibits excellent viewing angle characteristic and hence, the IPS liquid crystal display device is suitable also as an automobile-use display. As document which discloses such an IPS liquid crystal display device, JP-A-9-105918 (patent document 3) is named.
In the technique disclosed in patent document 1, a heater is provided by forming an electrode on a conductive plastic sheet, and by supplying an electric current to the conductive plastic sheet. Then, the conductive plastic sheet is adhered to the liquid crystal display device so as to heat the liquid crystal display device under low temperature thus overcoming a drawback relating to the temperature environment. This technique is characterized by uniformly heating the liquid crystal display device.
However, in the technique disclosed in patent document 1, it is necessary to additionally mount the conductive plastic sheet as a separate part and hence, there arise drawbacks such as a rise of cost attributed to the increase of number of parts or the increase of a step for assembling the conductive plastic sheet in the liquid crystal display device. The response characteristic of the liquid crystal display panel requires the elevation of temperature of liquid crystal sandwiched between two glass plates to a temperature suitable for an operation of a liquid crystal layer. With respect to the technique disclosed in patent document 1, there exists a possibility that heat generated by the heater is not efficiently transmitted to the liquid crystal portion depending on a location where the conductive plastic sheet is mounted.
In the technique disclosed in patent document 2, a touch panel is mounted on a display screen of a liquid crystal display panel, a temperature of the touch panel is detected by a separately-mounted temperature sensor. When the temperature is low, electrodes of the touch panel are also used as a heater thus overcoming a drawback relating to a temperature environment. However, in the technique disclosed in patent document 2, the touch panel is firstly heated to elevate the temperature of the touch panel, and an operation of the touch panel has to be performed only after heat is transmitted to the liquid crystal display panel. Accordingly, there arises a drawback that it takes time to start the liquid crystal display device from the low-temperature environment.